This one pretty clearly keys to Kogotus, but it also looks fairly different from specimens I caught in the same creek about a month later in the year. With only one species of the genus known in Washington, I'm not sure about the answer to this ID.
About "Mystery Creeks": If you recognize one of these, you already understand why I'm keeping it a secret. I'm not as strict as some anglers about hiding where I fish, mostly because I don't expect to substantially affect fishing pressure on already well-known or simply unpopular waters. But there are some gems where I don't want to add a single unfamiliar bootprint to the mix, due to the fishing, their wild character, or keeping a friend's secret. They're all "Mystery Creek" here—even the lakes.
This specimen is 14 mm. Technically this is the subspecies T. h. hecuba. The Cascades, Sierras and further West is where the other subspecies, T. h. pacifica is found. The Great Basin seems to have formed a barrier preventing any overlap in their distribution.